The invention relates to a photodetector, or optoelectronic sensor, and a method for detecting radiation.
To date, two principles for detecting radiation in solid-state image sensors prevail: charge-coupled-device (CDD) sensors and metal oxide semiconductor (MOS) sensors. CCD sensors are limited in dynamic range, because of the limited capacitance under the charge collecting gate of the CCD. Many applications, however, require a wide dynamic range signals of up to 140 dB in terms of the relationship between minimum and maximum detectable. In MOS sensors, particularly in active pixel sensors (APS) which use CMOS technology, this problem can be solved by using a detector producing a logarithmic response to the incident light intensity see reference [Cha 1]. But such a logarithmic response detector disadvantageously has a response time inversely proportional to the light intensity. This is a problem because many applications require a high readout speed over the whole dynamic range, including for very low radiation intensities, where classical logarithmic response detectors have an extremely long response time see reference [Vie 2].
In a classical logarithmic response photodetector, as shown schematically in FIG. 1, photodiode 100 is connected to the channel of a MOS transistor 102. The gate 104 of the transistor is connected to its drain. Light impinging on the photodiode generates a photocurrent Iph that is converted into a voltage over the MOS transistor. For typical photocurrents from femtoamperes to nanoamperes, the transistor operates in weak inversion and the voltage across the transistor VGS (gate to source voltage) can be calculated using formula 1.                               V          GS                =                                            kT              κq                        ⁢                          ln              ⁡                              (                                                      I                    ph                                                        I                    0                                                  )                                              +                      V            TH                                              Formula        ⁢                  xe2x80x83                ⁢        1            
In formula 1, xcexa is a process dependent transistor parameter, I0 is the drain current at the onset of the weak inversion operation of the transistor and VTH is the transistor""s threshold voltage. kT/q is roughly 26 mV at ambient temperature (k=Boltzmann""s constant, T=temperature and q=electronic charge).
To understand the response time of a logarithmic detector the equivalent circuit, or incremental model, of FIG. 2 can be considered. In this equivalent circuit, the loading MOS transistor is replaced by a transconductance 106, whose value gmlog depends on the photocurrent Iph, and the photodiode is replaced by an equivalent current source 107 which drains current from the transconductance 106. See formula 2.                               g          mlog                =                                            δ              ⁢                              xe2x80x83                            ⁢                              I                ph                                                    δ              ⁢                              xe2x80x83                            ⁢                              V                DS                                              ∝                                    I              0                        ⁢                          exp              ⁡                              (                                  V                  DS                                )                                                                        Formula        ⁢                  xe2x80x83                ⁢        2            
In formula 2, VDS is the drain to source voltage across the transistor.
Thus at a given light intensity, the response time of the detector to a small change of the photocurrent is given by formula 3, where Cph is the capacitance of the photodiode capacitor 108. This response time becomes excessively long for very small photocurrent values.                     τ        =                              C            ph                                g            mlog                                              Formula        ⁢                  xe2x80x83                ⁢        3            
Integrating CMOS photodetectors overcome the problem of the response time by performing a reset of the accumulated photocharge after readout. But since the response of these sensors is linear to the incident light intensity, such integrating sensors have a dynamic range limited by the voltage swing on the integration capacitance on the one hand, and the minimum detectable signal due to readout noise on the other hand.
The invention provides a photodetector, or optoelectronic sensor, and a method for detecting light as defined in the appended independent claims. Preferred or advantageous features of the invention are defined in dependent subclaims.
In order to realise a photodetector providing both fast response time and high dynamic range, the invention may advantageously provide a combination of aspects of the integrating and logarithmic photodetectors.
In a preferred embodiment of the photodetector, a photodiode (or other zone of semiconductor material for collecting photogenerated charges) and a capacitance are coupled in parallel between a ground line and a sense node, and the channel of a MOS transistor is coupled between the sense node and a supply voltage line. Usually, the capacitance may be the parasitic capacitance of the photodiode (or the zone of photosensitive semiconductor material) and neighbouring components. A low capacitance is usually desirable as this improves circuit sensitivity.
In an initial reset, the capacitance is charged so that the sense node voltage is above a transition voltage as described below.
After resetting, the transistor gate voltage is set so as to block, or switch off, the transistor, and the photodetector enters a sampling phase. Current flows through the photodiode in relation to the radiation intensity incident on it, which initially discharges the capacitance so that the sense node voltage falls substantially linearly in relation to the radiation intensity. At this stage no current flows through the transistor but if and when the sense node voltage falls to the transition voltage, the transistor enters its weak inversion operation domain. The photocurrent can then flow through the MOS transistor channel and so the sense node voltage begins to fall logarithmically in relation to the radiation intensity.
The transition voltage is equal to the gate voltage applied to the load transistor during sampling minus the transistor threshold voltage. Advantageously, the transition voltage can therefore be set by controlling the gate voltage or by adapting the threshold voltage by technological parameter adjustment.
In more general terms, it will be appreciated that the invention allows the load transistor to be used in both its normal mode, during which the photodetector response is linear, and in its weak inversion mode, during which the photodetector response is logarithmic. In addition, the invention ensures a smooth transition between these modes and makes it possible to choose the level at which the transition occurs.
In a further preferred implementation, the transition voltage may be varied during operation, such as during a sampling phase or between sampling phases, by varying the gate voltage of the load transistor. This may advantageously allow even more flexible control of the linear/logarithmic response of the photodetector and may allow the detector""s dynamic range to be further increased.
An important feature of preferred embodiments of the invention is therefore to be able to choose, or control, the sense node voltage at the beginning of the sampling phase and to choose, or control, the gate voltage of the load transistor during the sampling phase. These voltages determine the behaviour of the circuit and the transition from linear to logarithmic behaviour.
Thus, the invention may advantageously provide a photodetector and a method of detecting radiation which provide a much greater dynamic range and speed of response than conventional detectors. In particular, the invention may achieve a logarithmic response over part of its dynamic range without the drawback of increasing response time for low light intensities.
In a preferred embodiment, the sense node voltage after the initial reset may conveniently be close to the circuit supply voltage, achieved by coupling the sense node to the supply voltage line, for example by controlling the load transistor gate voltage to-switch on the load transistor channel.
In this document, the invention is embodied using a photodiode as a radiation sensitive element. In practice, however, a variety of types of component comprising zones of semiconductor material suitably doped to generate charges in response to incident radiation could be used.
In this document, examples are given in which the supply voltage is greater than the ground voltage. This could be reversed and the described circuits modified accordingly to create dual versions of the circuits, as the skilled person would be aware.
In practice, a photodetector of the invention may have a dynamic range of 120 dB or more, such as 160 dB. In tests, such circuits have achieved a measured dynamic range of about 200 dB.
The photodetector and method of the invention may advantageously find many applications, in particular for viewing applications with high contrast in the viewed scene. Such applications may include surveillance applications, soldering control, welding control or automotive driving assistance applications. Other applications would readily be apparent to the skilled person.